Air/fuel ratio feedback control system for lean combustion engine

A control system for feedback control of an air/fuel ratio of an air-fuel mixture supplied to an internal combustion engine which used a three-way catalyst for purifying the exhaust gas, the control system comprising:

• air/fuel ratio detection means for detecting actual values of air/fuel ratio in the engine;
• load detection means for detecting a load under which the engine is operating;
• transient condition detection means for detecting any of a plurality of predetermined transient operating conditions of the engine;
• and
• control means for performing feedback control of the feed of fuel or air to the engine based on the detected actual values of air/fuel ratio, the control means comprising target value setting means for determining a target value of the air/fuel ratio according to information obtained by said load detection means and said transient condition detection means said target value setting means being operable for setting the target value to a first value higher than the stoichiometric air/fuel ratio at least during predetermined steady-state operation of the engine and for shifting the target value to a second value selected to be optimum for the activity of the three-way catalyst when any of said predetermined transient operating conditions is detected;
• and
• modulation means for regulating the feed of fuel or air to the engine when said target setting means starts to shift the target value to said second value,
• said modulation means being operable for a predetermined time period at the start of the shifting of the target value for regulating said feed, and to deviate the target air/fuel ratio from said second target value in a direction opposite to the deviation of the first target value therefrom prior to shifting of the target value by said target value setting means.